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FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 6

FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 6

Auto-Ionization or Self-Ionization of Water:

Pure water is a very weak conclusion of electricity but the fact is that , water can conduct electric current, so there should be changed particles ( ions) in water.
The question arises that where do these ions come from?
This problem can be solved with the help of Lowery-Brorsted concept. Actually a water molecule releases a hydrogen ion (4+) ie proton and acid as an acid while this H+ is accepted by another H2O ion and thus this H2O molecule acts as a base (proton acceptor) this it is clear that H2O acts as an acid as well as, as a base, therefore we say H2O is amphoteric in nature

PH Scale:

In 1909, Soreson constructed a scale. Which is known as PH scale with the help of ph Scale the strength an acid or base is determined.
The term ph is refered as power of the hydrogen ions. The values of ph are from "O" to "14" The ph of acids is below "7". The stronger the acid the smaller will be its ph values and vice versa. Similarly the larger the value of ph a base the stronger will be that base and vice versa. The strength of bases can be determined by the tern POH. The values of POH also range from O to 14 but are in the reverse direction of values of ph

PH = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
POH 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

The strength of an acid is determined by determining its ph. The ph of an acid is the negative log of hydrogen ion (H+) concentration Ie
PH = -log [H+]

The strength of a base id determined by determining its POH. The POH of base is the negative log of  hydroxide ion concentration ie
Ph = -log [H+]

It is important to note that sum of the pH and pOH of a solution is always equal to 14
Ie pH + pOH = 14

The above relationship can be proved as consider the auto- ionization of water ie
2H2O ---------------> H3O + OH(+)
Here, the ionization of H2O is smaller that no considerable change occurs in the concentration of water and it still remains constant.

Solubility Product:

The term solubility product is used for the sparingly soluble salts in water. It can also be called as solubility product constant.
It can be defined as:
The solubility product or solubility product constant of sparingly soluble salt is defined as the product of molar concentration of its ion raised to the power of their stoichiometric co-efficient in the equilibrium equation:

Buffer Solutions:

The solution which resists change in its PH, even with the addition of small amount of a strong acid or a strong base is known as buffer solution.
A buffer solution is made as
1). When a weak acid ( like CH3COOH ) is mixed with its salt made by it with a strong base ( ie:  CH3COONa). They are mixed in a particular ration. This mixture is known as buffer and the solution of this mixture is known as buffer solution. Such a buffer is known as Acidic buffer. Ie CH3COOH/ CH3COONa buffer
2). When a weak base ( like NH4OH ) is mixed with its salt made by it with a strong acid ( ie NH4CL ) in affixed ratio and then dissolved in water. We get a buffer solution such a buffer is known as basic buffer
ie NH4OH / NH4CL . buffer The buffer solution has a particular PH range and this range remains constant even for long time or even after the addition of small amount of strong and or strong base.

Written by: Asad Hussain